Plastic fittings/water pipes and insulating underfloor heating beneath screed?

The insulation is fine for heating pipes but ...
Drinking water pipes must never be installed in a floor with [Fußbodenheizung].
The insulation doesn't help there, your cold water will always be warmed at the beginning and possibly full of germs.

It's not about the pipe losing heat, but that it cannot heat up.
However, this means an ambient temperature of about 24 °C.
Your [Fußbodenheizung] is about 30 to 35 °C.
Your cold water temperature will then also be that.

Sorry if my answer is too late.
Alternatively, install new pipes over the ceiling.

Yeah, sure - where is that supposed to be placed? And where do you want to put it? Connect it to the faucet via Wi-Fi?

Underfloor heating is in the subject line. What are you doing with cold water (30°C)?
At first you probably find it funny but then?!?

It stays cold - You don’t seriously believe that this will become a problem, do you? This is installed everywhere and is nowhere a problem. But go ahead, tell yourself how this should be solved alternatively?

?! Holger, where else should the water pipes be laid? As far as I know, there are distance regulations for hot water (and probably also for heating pipes) and insulation requirements for cold water. The water pipe will not be in the screed...

You can lay drinking water pipes in the screed, but only as long as there is no underfloor heating. I unfortunately do not understand what you want to tell me with the distance rules?